Chytridiomycosis infection

  • Fungal disease caused by Batrachochytrium dendrobatidis (Bd)
  • Bd is a fungus that lives in the gut of animals
  • Infects frogs, and has caused decline in over 500 amphibian species
  • Causes skin disease in the frogs, which is fatal

Image: New Scientist

Natural history

  • Bd fungus does not survive in warm climates (> 30 degrees)
  • Warm host climate also reduces lethality

Idea

If we can heat frogs, maybe we can reduce the impact of this infection

Experiment setup

  • Frogs separated into two classes: shaded and unshaded (heated)
  • Artifical refugia placed into each mesocosm (replicate)
  • Measure infection load and mortality

Image: Waddle et. al, Nature 2024

Hypothesis

  1. Do frogs in heated refugia experience a reduction in force of infection compared to non-heated refugia?
  2. Does prior infection confer an immune response that reduces future susceptibility?

Test using a stochastic compartmental model, with approximate Bayesian computation

Model setup

\lambda_h(t) = \beta_h \sum_{k=\{U,V\}}\sum_{j=1}^3 m_{k,j}I_{k,j},

  • m_{k,j} = Relative infectiousness of frog to I_{U,3}
  • I_{k,j} = Infection stage of frog

Parameters to estimate

Parameter Value
\beta_{SH}, \beta_{UN}, \alpha, \omega fitted
\mu 0.021
\gamma_1 1/2.5 per week
\gamma_2 1/4.5 per week
Parameter Value
m_{U,1} 10
m_{U,2} 100
m_{V,1} 1
m_{V,2} 10
m_{V,3} 0.1

Data

  • 8 mesocosms, 4 shaded 4 unshaded
  • 5 frogs per mesocosm
  • Some frogs experienced prior infection
  • Frogs observed at t=1, 2, 4, 6, 8, 10, 15 weeks only
  • Frogs that couldn’t be found presumed dead

Caution

For more data information, ask Claire Miller and/or read Waddle et. al Hotspot shelters stimulate frog resistance to chytridiomycosis.

Results

Parameter Prior Posterior (Mean, 95% CI)
\beta_\text{sh} \sim U(0,2) 0.675 (0.025, 1.876)
\beta_\text{un} \sim U(0,2) 0.401 (0.008, 1.767)
\alpha \sim U(0,0.5) 0.072 (0.003, 0.373)
\alpha\beta_\text{sh} 0.018 (0.001, 0.061)
\alpha\beta_\text{un} 0.009 (0.000, 0.031)
\omega \sim U(0,2) 0.569 (0.122, 1.688)

Posterior distributions

Predicted trajectories

Summary

  • Heating of artifical refugia can reduce infection by almost 50%

  • Prior infection highly protective (approx 97%)

  • Small numbers of frogs

  • Limited observation frequency

  • ABC-rejection not the most refined algorithm

Team

Claire Miller

Patricia Campbell

Jennifer Flegg

Experimental paper

Waddle et. al, Hotspot shelters stimulate frog resistance to chytridiomycosis Nature 2024

Code availability

Simulation-estimation experiment